The invention relates to a coating composition which contains solvents and, as binders, hydroxyl-containing acrylic polymers and aliphatic and/or cycloaliphatic polyisocyanate, may also contain flow-control agents, UV absorbers and radical traps and is intended in particular for producing, by the wet-on-wet method, a transparent topcoat of a multilayer coating which comprises at least one pigment-containing base coat and at least one transparent topcoat.
Two-layer metallic finishes are frequently produced using the wet-on-wet method: in a first step a base coat containing metal pigments with or without additional coloring pigments is produced by applying a base lacquer. After a period of air-drying, a transparent coating composition, ie. generally a clear lacquer, is applied as topcoat to the not fully hardened base lacquer, and the two coats are then jointly baked, ie. hardened. Also known are air-drying systems, which harden at room temperature.
The topcoat is frequently produced by means of clear lacquers which contain as binders hydroxyl-containing acrylate resins combined with amino resins. When ready for application, ie. when having a German Industrial Standard DIN 4 cup efflux time of 24 to 30 seconds, these clear lacquers normally have a solids content of 35 to 45% by weight and hence a solvent content of 55 to 65% by weight.
Environmental and economic reasons were behind the development of high-solid clear lacquers which, when ready for application, have a solids content of 45 to 55% by weight. The higher solids content is obtained by lowering the molecular weight of the binders and by using soft and hence more soluble binder components. The high-solid clear lacquers used to date have the disadvantage of adversely affecting the arrangement of the metal pigments of the base coats and hence of producing an unsatisfactory metallic effect, and the coatings produced from them have, in particular if low baking temperatures are used, an unsatisfactory resistance to chemicals.
Use is also made of two-component clear lacquers which contain as binders hydroxyl-containing acrylate resins and to which have been added polyisocyanates. These lacquers, when ready for application, customarily have a solids content of 32 to 45% by weight. Their advantages lie in a low hardening temperature of 20.degree. to 120.degree. C. and a higher resistance to chemicals. If an attempt is made to raise the solids content of these lacquers by increasing the proportion of acrylate resins having a low molecular weight, this gives rise to the problem of redissolving.